Ziegler-Natta type catalysts are well known and have been used since the early 1950's. Generally, the catalyst comprises a transition metal compound, typically titanium in the 3 or 4 valence state (e.g. TiCl3 or TiCl4) supported on magnesium chloride.
Some prior art catalysts are prepared by using an electron donor (ED), to dissolve the MgCl2 and TiCl3. When supported on silica, and placed in a gas phase reactor with at least one co-catalyst, typically an aluminum compound such as a trialkyl aluminum (e.g. triethylaluminum (TEAL)) or an alkyl aluminum halide (e.g. diethylaluminum chloride (DEAC)) this combination makes a good catalyst for the polymerization of polyethylene. The ED used as the solvent in the formulation tends to narrow the molecular weight distribution in the resulting polymer. As the ED is difficult to remove, polymers having a broader MWD are difficult to manufacture using this catalyst synthesis process. Also, as the ED must be capable of dissolving the MgCl2 and TiCl3, the choice of the electron donor is limited. A good description of these types of catalysts is contained in U.S. Pat. No. 4,302,566 issued Nov. 24, 1981 to Karol et al., assigned to Union Carbide Corporation. The Karol '566 patent also discloses the preparation of ethylene-butene copolymers in a gas phase polymerization process using the catalysts. The resulting ethylene-butene copolymers are a mixture of three different types of polymer fractions, namely a first fraction that is characterized by having a high molecular weight and having little or no butene comonomer; a second fraction of intermediate molecular weight and an intermediate level of butene comonomer; and a third fraction having a low molecular weight and a comparatively high amount of comonomer. In addition, there is a fourth typical feature of these copolymers, namely that the second fraction (i.e. the fraction having an intermediate molecular weight and intermediate level of comonomer) has a non uniform level of butene incorporation. In particular, the number of short chain branches—or “SCB” that result from the incorporation of the butene comomomer—decreases as the molecular weight increases. These ethylene-butene copolymers have enjoyed wide spread commercial success for more than 30 years. Several billion pounds of these copolymers are still being sold every year. For convenience, this type of product is referred to herein as a “Conventional Butene Resin”.
More recently, NOVA Chemicals Ltd. U.S. Pat. No. 6,140,264 issued Oct. 31, 2000 and U.S. Pat. No. 6,046,126 issued Apr. 4, 2000 to Kelly et al., both deal with making a TiCl4 supported catalyst on magnesium chloride (precipitated from a dialkyl magnesium compound and an organic halide) which is on a thermally and chemically treated silica. The technology used to synthesize such catalysts was further advanced in the manner disclosed in U.S. Pat. No. 7,211,535 (Kelly et al.). The catalysts disclosed in the Kelly '535 patent are highly active for the preparation of both ethylene-hexene copolymers and ethylene-butene copolymers in a gas phase polymerization process. The ethylene-butene copolymers that are normally produced with these catalysts (i.e. when using standard/conventional operating conditions in the gas phase process) are somewhat different from the Conventional Butene Resins described above. In particular, “new” ethylene butene resins that are normally produced with these catalysts are different from Conventional Butene Resins in two respects:
a) there is more comonomer in the very low molecular weight fraction of the Conventional Butene resins; and
b) the comonomer content/SCB distribution of the “new” resins is more uniform in the fraction of intermediate molecular weight.
These new resins have also enjoyed great commercial success, with hundreds of millions of pounds being sold each year since 2006. The new resins are generally considered to be an improved product (as both of the differences in resin architecture tend to provide some advantages for most customers). However, there is still a large market demand for Conventional Butene Resins.
We have now discovered a process that allows the production of the old Conventional Butene Resins using the new Kelly et al. catalysts. Thus, the same reactor may be used to produce both of the “new” resins and the conventional butene resins by operating the reactor under different conditions.